1. Field of the Invention
The present invention relates to technology for non-volatile storage.
2. Description of the Related Art
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in the programmed state. More information about programming can be found in U.S. Pat. No. 6,859,397, titled “Source Side Self Boosting Technique For Non-Volatile Memory;” and U.S. Pat. No. 6,917,542, titled “Detecting Over Programmed Memory,” both patents are incorporated herein by reference in their entirety.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states, an erased state and a programmed state that correspond to data “1” and data “0.” Such a device is referred to as a binary or two-state device.
A multi-state flash memory cell is implemented by identifying multiple, distinct allowed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits. The specific relationship between the data programmed into the memory cell and the threshold voltage ranges of the memory cell depends upon the data encoding scheme adopted for the memory cells. For example, U.S. Pat. No. 6,222,762 and U.S. Patent Application Publication No. 2004/0255090, both of which are incorporated herein by reference in their entirety, describe various data encoding schemes for multi-state flash memory cells.
Typically, the program voltage (Vpgm) is applied to the control gates of the memory cells as a series of pulses. The magnitude of the pulses is increased with each successive pulse by a predetermined step size (e.g. 0.2v, 0.3v, 0.4v, or others). In the periods between the pulses, verify operations are carried out. That is, the programming level of each memory cell of a group of cells being programmed in parallel is sensed between each programming pulse to determine whether it is equal to or greater than a verify level to which it is being programmed. One means of verifying the programming is to test conduction at a specific compare point. The memory cells that are verified to be sufficiently programmed are locked out, for example, by raising the bit line voltage to stop the programming process for those memory cells. The above described techniques, and others described herein, can be used in combination with various boosting techniques to prevent program disturb and with various efficient verify techniques known in the art.
When programming, there is a tradeoff between speed of programming and precision of programming. The precision of programming is related to the distribution of threshold voltages of the programmed memory cells subsequent to the programming process. The tighter the threshold voltage distribution, the easier it is to unambiguously read the memory cells. The need for tight threshold voltage distributions is even more important with multi-state memory cells because the read process needs to unambiguously distinguish between the different threshold voltage distributions. To obtain a tight threshold voltage distribution, a smaller step size is used for the program voltage Vpgm. However, using a smaller step size slows down the programming process.